Widespread use of renewable energy (e.g., wind power) is coming; the only question is how long it will take to get here. Berger (Restoring the Earth) argues persuasively that at present, the limiting factor is political rather than technological. Huge governmental subsidies for the oil, coal and nuclear industries, coupled with massive environmental costs paid by society rather than users, make it difficult for renewable energy sources to gain a major foothold. But even so, he explains, each year an increasing percentage of the world's energy supply arises from non-polluting renewables. Berger focuses on the promises offered by four alternative energy technologies--solar power, wind power, bioenergy and geothermal energy--as well as on energy conservation strategies, with solar energy and electric cars drawing the bulk of his attention. At times encyclopedic, his book is an amalgam of telling insights into the leaders of upstart companies and the problems they face plus detailed descriptions of apparatuses that only a techie could love. Photos. Rights (except electronic): Virginia Barber. (May)